Interhemispheric functional connectivity asymmetry is distinctly affected in left and right mesial temporal lobe epilepsy

Abstract Introduction The differences of functional connectivity (FC) and functional asymmetry between left and right mesial temporal lobe epilepsy with hippocampal sclerosis (LMTLE and RMTLE) have not been completely clarified yet. The purpose of the present study is to investigate the FC changes and the FC asymmetric patterns of MTLE, and to compare the differences in FC and functional asymmetry between LMTLE and RMTLE. Methods In total, 12 LMTLE, 11 RMTLE patients, and 23 healthy controls (HC) were included. Region of interest (ROI)‐based analysis was used to evaluate FC. The right functional connectivity (rFC) and left functional connectivity (lFC) of each ROI were calculated. Asymmetry index (AI) was calculated based on the following formula: AI=100×(rFC−lFC)/[(rFC+lFC)/2]. Paired t‐test and univariate analysis of variance were used to analyze FC asymmetry. Linear correlation analysis was performed between significant FC changes and lateralized ROIs and epilepsy onset age and duration. Results LMTLE and RMTLE patients showed different patterns of alteration in FC and functional asymmetry when compared with controls. RMTLE presented more extensive FC abnormalities than LMTLE. Regions in ipsilateral temporal lobe presented as central regions of abnormalities in both patient groups. In addition, the asymmetric characteristics of FC were reduced in MTLE compared with HC, with even more pronounced reduction for RMTLE group. Meanwhile, ROIs presented FC AI differences among the three groups were mostly involving left temporal lobe (L_hippo, L_amyg, L_TP, L_aMTG, and L_pTFusC). No correlation was found between significant FC changes and lateralized ROIs and epilepsy onset age and duration. Conclusion The FC and asymmetric features of MTLE are altered and involve both the temporal lobe and extra‐temporal lobe. Furthermore, the altered FC and asymmetric features were distinctly affected in LMTLE and RMTLE compared to controls.


INTRODUCTION
Mesial temporal lobe epilepsy (MTLE) is one of the most common types of refractory focal epilepsy in adults, and hippocampal sclerosis (HS) is the hallmark of most cases (Cendes et al., 1993). Previous studies have provided evidence that MTLE involved a complex pattern of brain functional changes that are not only confined to temporal lobes but also extended beyond the temporal regions (de Campos et al., 2016;Pereira et al., 2010). Importantly, the functional alterations in MTLE showed asymmetric patterns whereby abnormalities were more obvious in the hemisphere ipsilateral to the epileptogenic focus than the contralateral hemisphere (Pereira et al., 2010;Pittau et al., 2012). Pittau et al. (2012)reported that impairment of functional connectivity (FC) involved both the affected hippocampus and the contralateral "healthy" hippocampus, and the impairment was less severe for the "healthy" hippocampus compared with the affected one. Moreover, some studies showed that MTLE with left HS (LMTLE) presented different functional abnormalities compared with MTLE with right HS (RMTLE) (Besson et al., 2014;de Campos et al., 2016;Li et al., 2015). Li et al. (2015) observed alterations of both sides of the hippocampal network in unilateral MTLE, while RMTLE presented with stronger correlations between intra-hippocampus FC and disease duration than LMTLE. de Campos et al. (2016) showed that LMTLE was associated with more intricate bilateral dysfunction compared with RMTLE. In addition, LMTLE showed differential effects on cognitive function compared with RMTLE (Doucet et al., 2013;Kim et al., 2003). Verbal memory was mostly affected by LMTLE, whereas visuospatial memory impairment was more prominent in RMTLE (Pereira et al., 2010).
The above evidence suggests that HS may have different effects on the pathogenic mechanisms of MTLE depending on whether it is located in left or right hemisphere.
Structural and functional asymmetries have been well documented in the human brain and are thought to be a central principle of nervous system architecture that shapes the functional organization of most cognitive systems (Ocklenburg et al., 2016;Toga & Thompson, 2003). The normal asymmetric pattern in the human brain is beneficial for high-level functioning, including language, motor abilities, visuospatial attention, face processing, and reasoning (Gotts et al., 2013;Kong et al., 2018). On the contrary, changes in the normal hemispheric lateralization pattern have been associated with many cognitive and neuropsychiatric diseases (Carper et al., 2016;Zhao et al., 2019).
For instance, the generally attenuated FC asymmetry in schizophrenia patients increased with disease duration and correlated with psychotic symptoms (Ribolsi et al., 2014). In addition, the altered functional asymmetry of the primary sensorimotor cortex in patients with benign epilepsy and centrotemporal spikes was associated with intelligence quotient scores (Cao et al., 2017). Jung et al. (2018) showed that the FC asymmetry between the amygdala and the intraparietal sulcus was associated with severity of social anxiety symptoms. Furthermore, Gotts et al. (2013) provided direct evidence that functional asymmetry was associated with cognitive ability. Thus, altered functional asymmetry was thought to be correlated with cognitive function in numerous neuropsychiatric diseases.
Due to the importance of HS location in MTLE and the asymmetric characteristics of the human brain, we hypothesized that the differences between LMTLE and RMTLE may be related to the functional asymmetric features of the human brain. Understanding the unique effects of MTLE on FC and functional asymmetries can provide insights into the pathological processes associated with MTLE. However, the exact mechanisms have not been completely clarified. Resting-state functional connectivity magnetic resonance imaging (rs-fMRI) is a powerful method to identify plausible FC alterations for the reference region of interest (ROI) in neuropsychiatric disorders, by measuring the correlations of the blood oxygen level dependent (BOLD) signals between different brain regions (Buchbinder, 2016). Therefore, in this study, we used rs-fMRI to analyze the FC features of both left and right hemispheres as well as the functional asymmetric features of MTLE by directly comparing the FC of the left hemisphere to the right hemisphere. We hypothesized that not only the FC of MTLE but also the functional asymmetries of MTLE would be altered compared with HC, and the FC and functional asymmetries would be distinctly affected in the left and right MTLE.

Subjects
We recruited  Patients with lesions other than HS, mismatch between seizure semiology, v-EEG, and neuroimaging results and other neurologic disorders were also excluded. Among these participants, 11 had pathologically confirmed HS by the subsequent surgical treatment, and the other 12 cases of HS were diagnosed according to typical MRI findings (ipsilateral hippocampus atrophy, disturbed internal structure, increased signal intensity on T2FLAIR, and widening of ipsilateral temporal angle) (Malmgren & Thom, 2012). The age of epilepsy onset (first unprovoked seizure) and epilepsy duration were also recorded. In addition, 23 righthanded, healthy adults with matched age and gender were recruited as controls. There were no lesions on MRI and no self-reported cen-

Data analysis
Preprocessing of the rs-fMRI data was performed using CONN toolbox components each), scrubbing, motion regression (default 12 regressors), and filtering was used. aCompCor was shown to improve the specificity of functional connectivity estimates (Muschelli et al., 2014).
Subsequently, the data were parcellated into 105 cortical and subcortical regions using the FSL Harvard-Oxford Atlas provided with the toolbox. These ROIs were used as seeds of interest for ROI-to-ROI

Clinical data
The clinical and demographic characteristics of the three groups are summarized in Table 1 and Table S1. There were no significant differences with regards to gender (p = .503) or age (p = .051) among the LMTLE, RMTLE, and HC groups. In addition, no significant differences were found in epilepsy duration (p = .976) or epilepsy age of onset (p = .470) between the LMTLE and RMTLE groups.    and Table S3).  (Figure 1c and Table S4).

Functional connectivity asymmetry in the three groups
In the HC group, among the 37 ROIs (ROIs located in the midline of the brain were excluded in the asymmetric analysis), the FC of F I G U R E 1 Visualization of different functional connectivity between LMTLE and HC (a), between RMTLE and HC (b), and between LMTLE and RMTLE (c), respectively. Connections are depicted if significant at P FDR < 0.05 to represent meaningful findings and trends. Height of the T values is indicated by color as shown in the color bar. AG, angular gyrus; aMTG, anterior division of middle temporal gyrus; aPaHC, anterior division of parahippocampal gyrus; aSMG, anterior division of supramarginal gyrus; aTFusC, anterior division of temporal fusiform cortex; FDR, false discovery rate; FO, frontal operculum cortex; FP, frontal pole; IC, insular cortex; ICC, intracalcarine cortex; LG, lingual gyrus; LMTLE, left mesial temporal lobe epilepsy with hippocampal sclerosis; MedFC, frontal medial cortex, MidFG, middle frontal gyrus; PC, posterior division of cingulate gyrus; pMTG, posterior division of middle temporal gyrus; pPaHC, posterior division of parahippocampal gyrus; pSMG, posterior division of supramarginal gyrus; pTFusC, posterior division of temporal fusiform cortex; RMTLE, right mesial temporal lobe epilepsy with hippocampal sclerosis; ROI, region of interest; SCC, supracalcarine cortex; SFG, superior frontal gyrus; SPL, superior parietal lobule; sLOC, superior division of lateral occipital cortex; TP, temporal pole were observed between rFC and lFC. In total, 10 ROIs showed rightward asymmetry (rFC > lFC), and 6 ROIs showed leftward asymmetry (lFC > rFC). In the LMTLE group, the number of ROIs that showed asymmetric features was reduced to 12 ROIs (rightward asymme-try/leftward asymmetry: 4/8). Only one ROI (R_TP) (rightward asymmetry/leftward asymmetry: 1/0) in RMTLE showed FC asymmetry after multiple correction. Specifically, some ROIs showed leftward asymmetry in the HC group were reduced in the patient groups, such as the left frontal pole (L_FP), L_AG, L_sLOC, while some rightward asymmetry (R_SPL, R_pSMG, L_aSMG, L_ICC, L_LG, and L_IC) were reduced as well. In addition, the FC of the L_hippo, L_amyg, L_aMTG, R_ICC, and R_SCC were symmetric in the HC group, whilst showing asymmetric characteristics in the LMTLE group (Figure 2).

Correlation between abnormal functional connectivity and asymmetry and clinical variables of epilepsy
Pearson's correlation analyses were conducted to examine associations between the FC of significant abnormalities and AIs of significant lateralized ROIs and clinical features in LMTLE and RMTLE, respectively. However, no significant correlation was found between the FC and AI and the age of onset and epilepsy duration in both groups after multiple correction.

DISCUSSION
In this study, we analyzed the FC features and the functional asymmetry of the two hemispheres in patients with MTLE using rs-fMRI. Our study found that patients with MTLE had abnormal restingstate FC that are not limited to the epileptogenic zone, but involving widespread regions of the brain compared with controls. Specifically, the ROIs in the ipsilateral temporal lobe, such as the hippocampus, amygdala, aMTG, TP, PaHC, and TFusC stood out as central regions of abnormal FC. The FC asymmetry of majority of these regions was also changed in patients with MTLE. It was expected because the epileptogenic zone is located in the temporal lobe and thus caused the most pronounced impairment within temporal lobe, as many previous studies have shown (Bettus et al., 2010;de Campos et al., 2016;Li et al., 2015;Pereira et al., 2010). Such FC abnormalities of MTLE also could be a biomarker of epileptogenic zone localization. Furthermore, both LMTLE and RMTLE groups showed decreased FC between bilateral temporal lobe and related areas. The reduced FC of interhemispheric connections might provide adaptive inhibition to protect contralateral areas in seizure propagation (Sirin et al., 2020). The increased FC between regions within ipsilateral temporal lobe and between regions in ipsilateral temporal lobe and bilateral IC, FO, and SPL possibly reflected compensatory mechanisms (Bettus et al., 2009). In addition, some ROIs located in the frontal lobe, parietal lobe, and occipital lobe also showed FC abnormalities and different asymmetric characteristics among the three groups. This supports the theory that MTLE is a network disease that could involve broad areas other than the temporal lobe (Bernhardt et al., 2011;Pittau et al., 2012).
The FC between the two hemispheres was asymmetric in the HC group presented in this study in congruence with several previous studies (Badzakova-Trajkov et al., 2010;Toga & Thompson, 2003). Compared with HC, the asymmetric features of FC were reduced in both patient groups. For instance, the rightward asymmetry of the SMG presented in the HC group was reduced in the patient groups. The SMG was found to be involved in verbal working memory (Deschamps et al., 2014). Therefore, the reduced asymmetry of SMG in the patient groups may be associated with memory impairments and/or oroalimentary automatisms that are frequently observed in MTLE. Given that we did not test the relationship between these FC abnormalities and asymmetric features and cognitive performance, the clinical relevance of these findings can only be speculative. Nevertheless, the hemispheric asymmetry of certain functions such as language, spatial attention, and memory in the human brain was thought to be beneficial for functioning (Badzakova-Trajkov et al., 2010;Habib et al., 2003;Toga & Thompson, 2003). It is also known that distinct functions and a division of labor between the left and right hemispheres may improve overall cognitive ability and performance (Gotts et al., 2013 (Chiang et al., 2014). The limbic system is a complex neural circuit predominantly involved in memory and emotional output and has repeatedly been shown to affect TLE (Chan et al., 1997;Liacu et al., 2012;Zhao et al., 2019). The majority of studies showed bilateral limbic system impairment in TLE (Concha et al., 2005;Liacu et al., 2012), but few analyzed the asymmetric alterations of the limbic system. In our study, the functional AI of left limbic system was different between LMTLE and RMTLE, mostly due to the reduced rFC of these regions in LMTLE. It probably reflected the reduced compensation of left limbic system in LMTLE, because it was reported that the left hemisphere has a greater preference for within-hemisphere interactions and interacts less with the right hemisphere (Gotts et al., 2013;Li et al., 2015). Thus, when impairment involved the left limbic system, the compensatory pathway could not be as effective as the right limbic system. Taken together, these data provide evidence that the bilateral limbic systems play different roles in the communication and compensatory mechanisms associated with the bilateral sides of the brain.
There are some limitations to this study. First, the patient population was relatively small. Therefore, the findings should be interpreted with caution and future studies with more patients are required. Second, due to the lack of cognitive evaluations, the clinical relevance of these findings is only speculative. However, the previous study found that some cognitive functions such as language are associated with brain asymmetry (Sarica et al., 2018;Toga & Thompson, 2003). We would include cognitive evaluations in our future study. Third, we did not conduct fine screening of epilepsy comorbidities of the patients.
Some conditions comorbid with epilepsy, such as autism, are known to be associated with functional asymmetry. We should record details about epilepsy comorbidities in the future.

CONCLUSION
In conclusion, the abnormal FC and asymmetric features of MTLE are altered and involve both the temporal lobe and extra-temporal lobe.
Furthermore, the altered FC and asymmetric features were distinctly affected in LMTLE and RMTLE.